1. Field of the Invention
The present invention relates to a semiconductor laser device exhibiting a prolonged duration of operating life, which is advantageously used as an optical transmitter for the optical communication.
2. Description of the Prior Art
A semiconductor laser device basically has a device structure such that it comprises a semiconductor multi-layer film having optical confinement layers formed on both surfaces of active layers, and electrodes which are respectively formed on the upper surface and the lower surface of the semiconductor multi-layer film. The semiconductor multi-layer film is formed by successively laminating a plurality of semiconductor layers (compound semiconductor layers) having different compositions on a predetermined semiconductor substrate using, for example, epitaxial growth. Then, the semiconductor laser device constitutes an optical cavity relative to the laser pumped in the active layer by cleaving the above-mentioned semiconductor multi-layer film containing the electrodes in the direction perpendicular to the junction planes of the individual layers therein and allowing the opposite cleaved planes to function as ends of optical cavity.
Further, in general, one of the above-mentioned ends of optical cavity (cleaved planes) is coated with a high reflection film, and the other end of the optical cavity is coated with a low reflection film. These reflection films not only adjust the reflectance on the optical cavity end relative to the laser pumped in the active layer but also play a role of protecting the cleaved planes. The above low reflection film is formed as a single-layer film comprised of an oxide material (dielectric material) having a low refractive index, for example, aluminum oxide, silicon oxide or the like. In addition, the high reflection film is formed as a composite film obtained by alternately laminating, for example, the above-mentioned low reflection film and a film comprised of a material having a high refractive index, such as Si or the like. Such reflection films are formed by vapor deposition generally using a sputtering process.
By the way, when a semiconductor laser device having the above-described device structure is driven at a constant current, the optical output lowers with the lapse of time, and the lasing is finally stopped. Such a phenomenon is caused by a number of factors, and one of the factors is a problem of catastrophic optical damage.
This catastrophic optical damage is a phenomenon caused by the increased non-radiative recombination due to oxidation of the ends of the optical cavity during the driving of the semiconductor laser device. Such a phenomenon considerably lowers, for example, the driving reliability of the semiconductor laser device used as an optical transmitter for the optical communication, and thus, the improvement thereof has been strongly desired. Especially when the active layers and the semiconductor layers (optical confinement layers, and the like) near the active layers are comprised of an Al-containing compound semiconductor material, and in addition, the low reflection film (protection film) formed on the optical cavity end is an Al2O3 film, a problem arises in that the above-described catastrophic optical damage markedly occurs.
In the course of the studies made for solving the above problems, with respect to the phenomenon wherein, in the case where an Al2O3 film is deposited on the optical cavity end (cleaved plane) of the semiconductor laser device as a protection film (reflection film) therefor, when the active layers and the semiconductor layers near the active layers are comprised of an Al-containing compound semiconductor material, a catastrophic optical damage markedly occurs, the present inventors have made the following observations.
(1) When an Al2O3 film is deposited by a sputtering process which is generally employed as a method for depositing a protection film, the composition of the resultant film does not necessarily have a stoichiometric ratio. Rather, the Al2O3 film may contain an oxygen component in a stoichiometrically excess amount.
(2) In such a case, the excess amount of the oxygen component in the Al2O3 film is liberated due to the heat generated during the driving of the semiconductor laser device and the like, and diffused toward the cleaved plane side. Thus, the oxygen component diffused toward the cleaved plane side oxidizes the Al component of the Al-containing compound semiconductor material constituting the optical cavity ends (cleaved planes). It is considered that, as a result, a catastrophic optical damage occurs in the optical cavity.
(3) When the Al2O3 film stoichiometically lacks the oxygen component, the amount of the metal Al component becomes large, so that a current easily flows through the Al2O3 film when an electric field is applied thereto. Thus, this current causes the metal Al component and the Al component of the Al-containing compound semiconductor material constituting the optical cavity ends (cleaved planes) to undergo oxidation, so that a catastrophic optical damage easily occurs in the optical cavity similarly to the above case.
(4) Therefore, by using, as the Al2O3 film deposited on the optical cavity end, one that has the oxygen component which is not in a stoichiometrically excess amount, namely, has a composition such that the Al component and the oxygen component approximate to the stoichiometric ratio as closely as possible, the diffusion of the oxygen component and the oxidation of the Al-containing compound semiconductor material caused by the diffusion can be suppressed. Further, it has been considered that, by suppressing the oxidation of the Al-containing compound semiconductor material, it is possible to suppress the occurrence of a catastrophic optical damage in the optical cavity.
(5) In addition, it is considered that, by using, as the Al2O3 film deposited on the optical cavity end, one that does not stoichiometically lack the oxygen component, namely, has a composition such that the Al component and the oxygen component approximate to the stoichiometric ratio as closely as possible, the oxidation of the Al-containing compound semiconductor material is suppressed, thus making it possible to suppress the occurrence of a catastrophic optical damage in the optical cavity.
Based on the above observations, the present inventors have made various studies on the deposition of an Al2O3 film. As a result, it has been found that, when an Al2O3 film is deposited on the end of the optical cavity by, for example, the below-mentioned electron cyclotron resonance sputtering (hereinafter, referred to as xe2x80x9cECRxe2x80x9d) process, the resultant Al2O3 film is a film having a stoichiometric ratio composition. It is considered that, since sputtering is performed using a pure metal (Al) as a target in an oxygen atmosphere in the ECR process, the sputtered pure metal (Al) adheres to the optical cavity end while incorporating thereinto the required amount of oxygen in the atmosphere, so that the Al2O3 film deposited on the optical cavity end has a stoichiometric ratio composition. On the contrary, it is considered that, in the conventionally known sputtering process, alumina (Al2O3) is used as a target, and therefore, the composition for the molecules of the alumina sputtered varies depending on the manner of sputtering, so that the oxygen component of the Al2O3 film deposited on the optical cavity end is stoichiometrically changed.
Further, it has also been found that the resistivity of the Al2O3 film deposited using the above-mentioned ECR process is a considerably high value, as compared to that of the Al2O3 film deposited by the general sputtering process. In addition, it has been found that the semiconductor laser device having an Al2O3 film deposited which has such a high resistivity and a stoichiometric ratio composition is advantageous in that a catastrophic optical damage is considerably suppressed and almost no lowering of the optical output occurs after driving for a long time, resulting in a semiconductor laser device having high driving reliability, and thus, the present invention has been completed.
Thus, the semiconductor laser device of the present invention is characterized in that, as the low reflection film and/or high reflection film with which the end of the optical cavity is coated, an Al2O3 film having a stoichiometric ratio composition and having a resistivity of 1xc3x971012 xcexa9xc2x7m or more is used. Further, the semiconductor laser device is characterized in that the above Al2O3 film is deposited by, for example, an electron cyclotron resonance plasma sputtering process.
Specifically, in the present invention, there is provided a semiconductor laser device comprising a semiconductor multi-layer film formed by laminating optical confinement layers and active layers so as to dispose each of the active layers between the optical confinement layers, wherein one of the opposite ends perpendicular to the junction planes of the individual layers in the semiconductor multi-layer film is coated with a low reflection film and the other of the ends is coated with a high reflection film, wherein the low reflection film contains a film comprised of at least Al2O3 having a resistivity of 1xc3x971012 xcexa9xc2x7m or more.
Preferably, the low reflection film is formed as a single layer. Alternatively, the low reflection film is formed as a film comprised of a plurality of layers. In this case, the plurality of layers are realized as a composite film formed from a film comprised of the above Al2O3 and a film which contains Si and has a refractive index higher than that of the Al2O3. Particularly, it is preferred that, as the film which has a refractive index higher than that of the Al2O3, one that is selected from the group consisting of Si, xcex1(amorphous)-Si and SiN is used.
Further, preferably, the high reflection film contains a film comprised of at least Al2O3 having a resistivity of 1xc3x971012 xcexa9xc2x7m or more. Particularly, the high reflection film is realized as a composite film formed from a film comprised of the above Al2O3 and a film which contains Si and has a refractive index higher than that of the Al2O3. Also in this case, it is preferred that, as the film which contains Si, one that is selected from the group consisting of Si, xcex1(amorphous)-Si and SiN is used.
In addition, in the present invention, there is provided a semiconductor laser device comprising a semiconductor multi-layer film formed by laminating optical confinement layers and active layers so as to dispose each of the active layers between the optical confinement layers, wherein one of the opposite ends perpendicular to the junction planes of the individual layers in the semiconductor multi-layer film is coated with a low reflection film and the other of the ends is coated with a high reflection film, wherein the low reflection film contains a film comprised of Al2O3 having a stoichiometric ratio composition.
Also in this invention, preferably, the low reflection film is formed as a single layer. Alternatively, the low reflection film is formed as a film comprised of a plurality of layers. In this case, the plurality of layers are realized as a composite film formed from a film comprised of the above Al2O3 and a film which contains Si and has a refractive index higher than that of the Al2O3. Particularly, it is preferred that, as the film which contains Si, one that is selected from the group consisting of Si, xcex1(amorphous)-Si and SiN is used.
Further, preferably, as the high reflection film, one that contains a film comprised of at least Al2O3 having a substantially stoichiometric ratio composition is used. Preferably, the high reflection film is realized as a composite film formed from a film comprised of the above Al2O3 and a film which contains Si and has a refractive index higher than that of the Al2O3. In this case, particularly, as the film which has a refractive index higher than that of the Al2O3, one that is selected from the group consisting of Si, xcex1(amorphous)-Si and SiN is used.
Further, the present invention is characterized in that the above-mentioned Al2O3 film is deposited by an electron cyclotron resonance plasma sputtering process, an electron beam evaporation process, or an electron beam sputtering process.